The present invention relates to a hydraulically operated continuously variable transmission, and more particularly to a hydraulically operated continuously variable transmission including a hydraulic pump coupled to an input shaft and a hydraulic motor coupled to an output shaft, the hydraulic pump and the hydraulic motor being interconnected by a hydraulic circuit.
There is known a hydraulically operated continuously variable transmission for use in an automobile, including an input shaft, a hydraulic pump having a pump cylinder coupled to the input shaft and a plurality of pump plungers disposed in the pump cylinder in an annular pattern around an axis of rotation of the pump cylinder, the hydraulic pump having an outlet port, an output shaft, a hydraulic motor having a motor cylinder coupled to the output shaft and a plurality of motor plungers disposed in said motor cylinder in an annular pattern around an axis of rotation of the motor cylinder, the hydraulic motor having an inlet port, a closed hydraulic circuit interconnecting the hydraulic pump and the hydraulic motor, a pump swash plate for reciprocally moving the pump plungers, a tiltable motor swash plate for rotating the motor cylinder in response to reciprocating movement of the pump plungers, the tiltable motor swash plate being tiltable through a continuously variable angle for continuously adjusting the stroke of reciprocating movement of the motor plungers.
As disclosed in Japanese Patent Publications Nos. 32-7159 and 56-50142, for example, such a continuously variable transmission is controlled such that the transmission ratio will be 1:1 while minimizing or eliminating the angle of inclination of the motor swash plate.
By breaking the hydraulic circuit which interconnects the outlet port of the hydraulic pump and the inlet port of the hydraulic motor, the pump plungers can be locked in the pump cylinder to drive the motor cylinder mechanically through the pump swash plate. With the hydraulic motor and pump thus mechanically locked, the oil pressure discharged by the hydraulic pump is prevented from acting on the motor plungers to reduce the thrust load on the motor swash plate and also to reduce oil leakage from between the motor plungers and the motor cylinder.
Where a shoe is mounted on the distal ends of the motor plungers and a hydraulic pressure introduced to the sliding surface of the shoe for balancing the hydraulic pressures, the hydraulic pressure on the sliding surface of the shoe can be lowered and any oil leakage can be reduced by breaking the hydraulic circuit.
It is known that it is possible to improve the power transmission efficiency of the transmission and increase the durability of the transmission by breaking the hydraulic circuit between the hydraulic pump and the hydraulic motor.
It is known that a hydraulically operated continuously variable transmission may have a passage capable of communication between the outlet and inlet ports of the hydraulic pump and also to have a clutch valve for varying the cross-sectional area of the passage to change the amount of power transmitted between the hydraulic pump and the hydraulic motor thereby to control power transmission between the input and output shafts.
Japanese Laid-Open Patent Publication No. 54-134252 discloses a common valve for closing and opening the outlet port of the hydraulic pump and for regulating the amount of oil flow through the passage between the outlet and inlet ports of the hydraulic pump. In such a disclosed arrangement, since the outlet port of the hydraulic pump is selectively closed and opened and the clutch valve is continuously operated or under analog control, delicate clutch control could not be achieved if the outlet port of the hydraulic pump and the clutch valve were controlled by a single actuator.
The making and breaking of the hydraulic circuit and the clutch valve are separately controlled by independent actuators in a transmission disclosed in Japanese Laid-Open Patent Publication No. 55-152622. However, if the outlet port of the hydraulic pump were opened at the time the clutch valve is controlled to make the clutch OFF in order to bring the transmission into a neutral condition, the hydraulic pump and the hydraulic motor would be mechanically coupled by the pump plungers and the pump swash plate, and the power would not be cut off even if the clutch is OFF.
Japanese Patent Publication No. 56-50142 discloses an arrangement for making and breaking the hydraulic circuit by providing a piston rod which can break the hydraulic circuit and applying a hydraulic pressure to the piston rod to break the hydraulic circuit.
The hydraulic circuit is made under the resiliency of a return spring, which should be capable of producing a relatively large spring force in order to make the hydraulic circuit reliably. An actuator employed for moving the piston rod to breaking the hydraulic circuit against the force of the return spring should therefore by capable of applying a large force and needs to be large in size. The disclosed arrangement is not preferable for this reason.
One solution is to use a hydraulic servomotor for operating a device to make and break the hydraulic circuit with a relatively small force, as disclosed in Japanese Laid-Open Patent Publication No. 54-134253. A valve for making and breaking the hydraulic circuit is operated by a pilot valve which is also governed by a spring force. In order to ensure reliable operation, therefore, a large spring force must be established, and the resulting system is large in size.
As disclosed in Japanese Laid-Open Patent Publications Nos. 54-134252 and 55-14312, for example, it is known that in order to achieve smooth and economical operation of a motor vehicle, it is effective to control the transmission so that the amount of opening of the throttle valve and the engine rotational sped will be proportional to each other.
When the hydraulic circuit is broken at the time the transmission ratio is minimal, since the volumetric efficiency becomes higher than before the circuit is broken, the engine rotational speed is lower as indicated by the following equations: ##EQU1##
Therefore, in a motor vehicle in which the transmission is controlled as a function of the throttle valve opening and the engine speed, the following steps are repeated if the throttle valve opening is kept at a substantially constant level:
(1) The transmission ratio becomes 1 (the angle of the motor swash plate is minimal); PA0 (2) The device for making and breaking the hydraulic circuit is operated to break the circuit; PA0 (3) The engine rotational speed is lowered due to an increase in the volumetric efficiency; PA0 (4) The angle of the motor swash plate is controlled so as to increase the transmission ratio; PA0 (5) The hydraulic circuit is established again; and PA0 (6) The transmission ratio becomes 1 again.
When the above steps are repeated in a short period of time, the device for making and breaking the hydraulic circuit is subjected to hunting, and smooth running of the vehicle may not be achieved.
The same problem is likely to happen when the transmission is controlled on the basis of the engine torque in order to obtain minimum fuel consumption. More specifically, when the engine rotational sped is reduced, the engine torque is increased, and the angle of the motor swash angle is controlled in the same manner as described above. The device for making and breaking the hydraulic circuit now suffers from hunting.